1. Field of the Invention
The present invention is directed in general to field of information processing. In one aspect, the present invention relates to a power de-rating reduction system and method in a transmitter.
2. Description of the Related Art
In general, coded orthogonal frequency division multiplexing (COFDM) systems support high data rate wireless transmissions using orthogonal channels, which offer immunity against fading and inter-symbol interference (ISI) without requiring implementation of elaborate equalization techniques. Typical COFDM systems split data into N streams, which are independently modulated on parallel spaced sub-carrier frequencies or tones. The frequency separation between sub-carriers is 1/T, where T is the COFDM symbol time duration. Each symbol may include a guard interval (or cyclic prefix) to maintain the orthogonality of the symbols. In general, COFDM systems have used inverse discrete Fourier transforms (IDFT) to generate a sampled (or discrete) composite time-domain signal, but such COFDM systems can exhibit relatively large peak-to-average power ratio (PAR) where there is constructive addition of signals from different sub-carriers. Large PARs and/or large cubic metrics (CM) are undesirable because they require a large dynamic range for a digital-to-analog (D/A) converter on the transmitter backend, and this in turn means that the D/A converter is inefficient since most sub-carrier amplitudes use a fraction of the range of the D/A converter.
In a typical transmitter backend, the output of the D/A converter is filtered before being applied to a power amplifier. Since power amplifiers tend to be non-linear, in-band distortion and spectral spreading (or spectral regrowth) may result from passing a band-limited time-varying (non-constant) envelope signal through the non-linear amplifier circuit. One technique for addressing non-linearity of a power amplifier is to operate the power amplifier at a relatively large output power backoff (OBO) or power de-rating, but this technique reduces the power efficiency of the amplifier. For example, at a 6 dB OBO, a power amplifier may exhibit a fifty percent (or more) loss in efficiency. To reduce the PAR and/or CM of COFDM systems, various designers have also implemented or proposed hard limiting (or clipping) directly on the signal to be transmitted. Unfortunately, directly clipping the signal to be transmitted may cause undesirable spectral regrowth and inter-user interference (or inter-carrier interference (ICI)) in systems that use a multiple access mode.
In certain wireless systems (such as the evolved-universal terrestrial radio access (E-UTRA) air interface), discrete Fourier transform-spread orthogonal frequency division multiplexing (DFT-SOFDM) has been proposed as a modulation technique for uplink transmissions. Single carrier transmission schemes, such as DFT-SOFDM, generally facilitate further power de-rating reduction through the use of specific modulation or coding schemes, clipping and/or spectral filtering of a signal to be transmitted. Moreover, the PAR and CM of a basic DFT-SOFDM or single carrier-frequency division multiple access (SC-FDMA) system is generally reduced, as compared to the PAR and CM of a basic COFDM system. To further reduce the PAR and CM of basic DFT-SOFDM transmitters, it has been proposed to pre-process an input signal prior to performing a fast Fourier transform (FFT) on a group of symbols associated with the input signal. Following this approach, selected input symbols and/or bits may be attenuated in order to reduce the PAR and CM at the output of an inverse fast Fourier transform (IFFT) of the DFT-SOFDM system.
FIG. 1 depicts a relevant portion of a prior art SC-FDMA system 100 that implements a quadrature phase shift keying (QPSK) modulation scheme. As shown, data bits from a serial-to-parallel converter 102 is applied to a plurality of bit-to-constellation mapping blocks 104 which map the data bits into IQ bits or symbols. In the SC-FDMA system 100, outputs of the mapping blocks 104 are provided as inputs to an M-point fast Fourier transform (FFT) block 114 and as inputs to an attenuator block 106. Outputs of the M-point FFT 114 and M additional zero values are also provided as inputs to a 2M-point inverse FFT (IFFT) block 116. Selected outputs of the 2M-point IFFT block 116 are provided to a peak detector 118 which finds a peak value that exceeds a defined threshold, and uses the peak value to control the attenuator block 106, which attenuates selected bits (or symbols) a desired amount according to signals provided at the control inputs of the attenuator block 106. Outputs from the attenuator block 106 are provided to a conventional SC-FDMA transmitter, such as the M-point FFT block 108, an N-point IFFT block 110 and a cyclic prefix (CP) block 112 which adds a desired guard band to each symbol group. It should be noted that the sub-carrier mapping block that maps the samples of the M-point FFT to N sub-carriers in a localized manner as shown here.
While the SC-FDMA system 100 exhibits improved power de-rating reduction over conventional SC-FDMA systems, there is a need for additional power de-rating reduction for a transmitter (e.g., an uplink transmitter) that further reduces inter-carrier interference and spectral regrowth and improves the transmit power efficiency, particularly with mobile user equipment devices. In addition, there is a need for an improved system and methodology for PAR reduction that complies with the uplink for third generation partnership project long term evolution (3GPP LTE). There is also a need for an SC-FDMA transmission scheme which overcomes the problems in the art, such as outlined above. Further limitations and disadvantages of conventional solutions will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.